1. Field of the Invention
The present invention relates to a light-emitting chip device; and more particularly to a light-emitting chip device with high light extraction efficiency and high thermal conductivity.
2. Description of the Related Art
Please refer to FIG. 1, which shows a conventional light-emitting chip 1. FIG. 1 includes a substrate 11, an epitaxial-layer structure 12 on the substrate 11 and an electrode unit 13 constituted of an N-type electrode 131 and a P-type electrode 132.
As an example, the epitaxial-layer structure 12 is formed of GaN-based material and has an N-type first cladding layer 121, an active layer 122 formed on the first cladding layer 121 and a P-type second cladding layer 123. The first cladding layer 121 and the second cladding layer 123 are opposite to each other and form carrier injectors relative to the active layer 122. As such, when electric power is provided to the epitaxial-layer structure 12, electrons and holes would be recombined together and then release energy in a form of light emission.
The N-type electrode 131 and P-type electrode 132, for example, are formed of Au, Ni, Pt, Ag, Al, etc. and/or their alloy. The N-type electrode 131 is disposed on and forms ohmic contact with the first cladding layer 121 of the epitaxial-layer structure 12. The P-type electrode 132 is disposed on and forms ohmic contact with the second cladding layer 123 such that the N-type electrode 131 and P-type electrode 132 provide electric power supply to the epitaxial-layer structure 12.
When electric energy is supplied to the N-type electrode 131 and P-type electrode 132, current spreads and flows through the epitaxial-layer structure 12, and electrons and holes are injected into the active layer 122, recombining with each other and releasing energy in the form of light emission.
The refractive index of the GaN-based material is about 2.6, and the refractive index of its surrounding, which generally is air, is 1, or the surrounding is a transparent encapsulating material, used for packaging and having a refractive index between 1 and 2.6. The top surface 124 of the second cladding layer 123 of the epitaxial-layer structure 12 of the light-emitting chip 1 is a flat surface. Partial light generated from the epitaxial-layer structure 12, due to their propagation direction, would follow Snell's law and could not escape the epitaxial-layer structure 12 and enter the surrounding. As a consequence, the light extraction of the light-emitting chip 1 is not good.
Please refer to FIG. 2. There are literature and patents that propose roughing the top surface 124′ of the light-emitting chip 1 to make the light impinging on the rough top surface 124′ have various incident angles relative to the rough top surface 124′. The chance of light escaping the epitaxial-layer structure 12′ is thus increased, and the light extraction efficiency is improved.
Nevertheless, the light generated from the epitaxial-layer structure 12′ does not entirely propagate toward the top surface 124′. The light propagating toward the substrate 11 faces similar situation as that at the top and can not escape the epitaxial layer 12′ and enter the surrounding. Thus, the light extraction is still low.
Please refer to FIG. 3. Some literature proposes to form a reflective mirror layer 111, which is connected to the epitaxial-layer structure 12′, capable of reflecting light. Hopefully, the light propagating toward the substrate 11′ can be reflected toward the top surface 124′ to improve the possibility of light generated from the epitaxial-layer structure 12′ to escape the epitaxial-layer structure and enter the surrounding. However, the light propagating toward the substrate 11′ would be confined in the epitaxial-layer structure 12′ due to their propagation directions and causes total internal reflection within the epitaxial-layer structure 12′. Furthermore, the light can be absorbed by the active layer 122. The reflective mirror layer 111 on the substrate 11′ cannot substantially improve the light extraction of the light-emitting chip 1. When a roughened interface is formed between the epitaxial-layer structure 12′ and the reflective mirror layer 111, and a low-refractive-index transparent material is added between them, the light entering the low-refractive-index transparent material from the epitaxial-layer structure 12′ is easily reflected back, and the roughened interface would easily change the propagation of the reflected light. The total reflection within the epitaxial-layer structure 12′ is eliminated. The light extraction thus can be increased.
Nevertheless, the N-type electrode 131 is disposed on the first cladding layer 121 and the P-type electrode 132 is disposed on the second cladding layer 123, both of them block some light emitted from the front side of the light-emitting chip 1, and resulting in the reduction of the light-emitting area. The brightness of the light-emitting chip 1 is lowered.
Besides, the internal waste heat converted from the light confined within the epitaxial structure 12′ is dissipated through the substrate 11′, and the dissipation efficiency is not good. The lifetime of the light-emitting chip 1 is adversely affected.